This invention relates to blowing processes for refining molten metal in a vessel. Particularly, the invention relates to top blowing processes for improving removal of carbon, such as in a basic oxygen process.
It is known to produce ferrous metals in molten metal vessels wherein top blowing with oxygen through a lance positioned above the bath is used. For this purpose the vessel is typically charged with 60 to 80% hot metal, for example, from a blast furnace and 20 to 40% of a cold charge which may be high-carbon chromium alloy and/or stainless steel scrap. Top oxygen blowing is performed until the final bath carbon level has been reduced to approximately 0.035 to 0.05%; at which time the bath temperature is typically 3400.degree. to 3600.degree. F. (1871.degree. to 1982.degree. C.). At such carbon content, which may be currently achieved by the use of a top-blown basic oxygen converter, the bath temperatures are sufficiently high that excessive refractory wear occurs. Presently, many product specifications require carbon levels less than 0.03%. The standard basic oxygen furnace practice cannot attain such low carbon levels.
It is also known, in top-blown oxygen steelmaking processes of this type, to blend an inert gas, such as argon, with the oxygen introduced by top blowing near the end of the blowing cycle. Although argon serves to improve the efficiency of the carbon removal, nevertheless, stainless steels having carbon contents less than about 0.03% may not be commercially produced on a consistent basis.
It has also been proposed to adapt a basic oxygen converter vessel for introduction of an inert gas to the bath from beneath the surface thereof by the use of tuyeres or porous plugs arranged on or near the bottom of the vessel. Concurrently filed, copending application Ser. No. 604,098, filed Apr. 26, 1984, discloses a method which comprises top blowing from a lance oxygen and/or a mixture or oxygen and inert gas onto or beneath the bath surface while introducing a low flow rate inert gas to the bath from beneath the surface during the top blowing. The overall ratio of oxygen-to-inert gas is decreased progressively during top blowing. The relative proportion of the top-blown gases and bottom inert gas remain substantially the same throughout the process.
There have been proposals by others to use top blowing of oxygen and bottom injection of inert gases. U.S. Pat. No. 3,325,278, issued June 13, 1967, discloses top blowing of oxygen onto the bath surface while concurrently introducing an inert gas in the lower portion of the bath at a rate no greater than the top oxygen flow rate. U.S. Pat. No. 3,854,932, issued Dec. 17, 1974, describes a method of top blowing oxygen while introducing an inert or endothermic gas through a bottom tuyere while maintaining a subatmospheric pressure in the converter. U.S. Pat. No. 4,280,838, issued July 28, 1981, discloses a method of top blowing oxygen and bottom blowing through tuyeres a gas predominantly carbon dioxide at a rate which is a small fraction of the rate of top-blown oxygen. Several other patents describe methods of top blowing oxygen and bottom blowing inert gas through tuyeres as a function of slag levels, such as U.S. Pat. Nos. 3,860,418; 4,325,730; 4,334,922; 4,345,746; and 4,369,060.
It is, accordingly, an object of the invention to provide a method for producing steel in a top-blown oxygen converter by simultaneously top blowing with oxygen and introducing inert gas from beneath the surface of the bath, wherein the rate of top-blown oxygen is progressively decreased as the rate of inert gas introduced beneath the bath surface is progressively increased.